The present invention relates to the field of brake shoes.
An air elevator is a platform constructed as an integral part of a missile launch system which supports the entire weight of the missile during loading and assembly of the missile in the launcher, and must be held at different positions along the length of the launch canister to allow work to be completed on various stages of the missile. The work platform is raised and lowered on a cushion of air and is held in position vertically by rubber lined, hydraulically or pneumatically actuated brake shoes that contact the inside surface of the launch canister. The static or breakaway coefficient of friction between the brake lining and the inside of the launch canister must be high enough to prevent slippage along the canister wall when the platform is loaded, yet must not develop adhesion forces that prevent brake retraction. Also the lining material must be soft enough to prevent chaffing of the canister wall during braking of the platform, since small movements in the platform are unavoidable during application of the brakes.
Of major concern was developing a material with high friction capabilities and little adhesion characteristics. This was a difficult task, since one major component of friction is the interfacial adhesion developed between the contacting surfaces. When two polymer surfaces are brought into contact under pressure, for extended periods of time, there is an opportunity for polymer chains and compound ingredients to diffuse from one material surface into the other. The result is high molecular forces between the materials, approaching the tensile strengths of the polymers themselves. This mechanism of friction is related somewhat to adhesion and is very undesirable for static breaking applications such as that of the air elevator. In this application the canister surface contacting the brake shoe, is coated with a polymer (polyurethane) paint.
It is thus an object of the present invention to provide:
a high friction rubber (brake shoe) formulation that exhibits low adhesion to other polymer materials as a means of developing high shear friction forces;
a high friction rubber formulation which exhibits a static friction coefficient in shear of up to about 1.5;
a high friction rubber formulation highly suitable for static braking applications; and
a high friction rubber formulation that exhibits low diffusion of compounding materials into a contacting polymer surface, thereby reducing molecular adhesion.